It is known to estimate the position of user devices by carrying out measurements of received signal strength from plurality of electromagnetic signal sources of known position (for example wireless access points, such as WiFi devices, Bluetooth enabled devices, radio beacons etc). Known techniques are especially useful indoors where global satellite navigation systems do not function reliably or at all.
In order to map a building for use with such a system, it is known to accurately measure the position of a plurality of electromagnetic signal sources in two or three dimensions, and then to systematically move through the building, carrying out scans at each of a plurality of accurately measured scanning points. At each point, a measurement is made of the received signal strength from each of the electromagnetic signal sources which can be detected at that location. The resulting data is stored in a database. During operation of the system, a user device scans for electromagnetic signal sources and then carries out measurements of received signal strength for each of the electromagnetic signal sources found in the scan. The resulting measurements are then compared with the data stored in the database and the position of the user device is assessed as being the position of the scanning point, from amongst the plurality of scanning points, where the measured data most closely corresponds to that stored in a database. It may also be possible to interpolate between scanning points. Known techniques to estimate the position of a user device in such a system include the use of fingerprinting techniques to compare the measured data with the data stored in the database.
Such methods have been found to be practically useful. However, the volume of data which must be stored in the database is prohibitive. Accordingly, the invention provides a method of obtaining estimates of the position of a user device which require less data to be stored.
US2007/0133487 describes a method of providing an estimate of a mobile location of a wireless node, point or terminal in a wireless local area network. A signal strength fingerprint algorithm is used to build up a signal strength database or radio map for a building. This involves measuring signal powers from wireless access points at a plurality of points in the building, and applying a linear regression algorithm to estimate mobile received signal strengths at a plurality of grid points within the building. In order to determine the mobile location of the wireless node, point or terminal, signal power received by the wireless node, point or terminal is correlated with the signal strength database or radio map to determine which grid point correlates most closely with the received signal powers. This would require the estimated received signal strengths at each grid point to be generated and stored in a memory for correlation with the received signal powers.